Themes: Make Add Setting button sensivite in all circumstances

This enabled the user to directly add a setting without having to add
a theme first.
3 jobs for themes-window in 2 minutes and 52 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #14215
run-appdata

00:00:07

passed #14214
run-tests

00:02:34

 
  Build
passed #14216
run-build

00:00:10